1. Field of the Invention
The present invention relates to a photovoltaic power generation system for maximizing the output power from a solar cell. In particular, it relates to a technique for precisely tracking the maximum output power among a plurality of local maximum powers resulting from uneven solar irradiation to a solar cell panel.
2. Description of the Related Art
FIG. 5 is a block diagram showing a conventional photovoltaic power generation system. In the figure, SC1 to SC3 designate three solar cells connected in parallel. PT designates an output voltage detector that detects the output voltage of the solar cells. CT designates an output current detector that detects the output current of the solar cells. CC designates a controller including a microprocessor that performs MPPT (Maximum Power Point. Tracking) control by using a hill climbing method (hereinafter referred to as “hill climbing method HC”). The controller CC includes an A/D converter into which the output voltage and the output current of the solar cells are input. IN designates a DC/AC inverter (or a DC/DC converter) that converts the DC output from the solar cells into a voltage. AD designates a load, and SP a commercially available power source for the system.
With the above arrangement, the microprocessor of the controller CC multiplies the output voltage and the output current of the solar cells SC to calculate the output power of the solar cells SC, and causes the output voltage and the calculated power to be stored in a memory. The microprocessor of the controller CC also outputs a “photovoltaic output setting value” for controlling the output of the DC/AC inverter IN via a pulse width modulator PWM, thereby controlling the output voltage of the solar cells SC.
FIG. 6 is a flowchart for explaining the hill climbing method employed in the conventional power generation system. The operation of the photovoltaic power generation system will be described below by using this flowchart, and FIG. 7 showing how the local maximum is tracked.
Specifically, a first photovoltaic output setting value, corresponding to the point A on the power-voltage characteristic graph shown in FIG. 7, is provided as an initial setting value from the controller CC (step S1) to operate the inverter in a certain mode. In this state, a first photovoltaic output voltage V1 from the solar cells SC is measured (step S2). Likewise, a first photovoltaic output current I1 is measured (step S3). Then, the microprocessor of the controller CC multiplies the measured voltage V1 and current I1 to calculate a first photovoltaic output power W1. The calculated output power W1 and the first photovoltaic output voltage V1 are stored in the memory (step S4).
Then, a second photovoltaic output setting value greater than the first photovoltaic output setting value is provided to operate the inverter, so that the first photovoltaic output voltage V1 shown in FIG. 7 is increased to a second photovoltaic output voltage V2 (step S5).
In this situation, a second photovoltaic output current I2 is measured (step S6). Then, the microprocessor of the controller CC multiplies the second photovoltaic output voltage V2 and output current I2 to calculate a second photovoltaic output power W2. Again, the calculated power W2 and the second photovoltaic output voltage V2 are stored in the memory (step S7).
Further, a third photovoltaic output setting value lower than the first photovoltaic output setting value is provided to operate the inverter, so that a third photovoltaic output voltage V3 lower than the first voltage V1 is obtained (step S8). In this situation, a third photovoltaic output current I3 is measured (step S9), and then the voltage V3 and the current I3 are multiplied to calculate a third photovoltaic output power W3. The calculated power W3 and the voltage V3 are stored in the memory (step S10).
The stored output powers W1, W2 and W3, corresponding to the output voltages V1, V2 and V3 (V3<V1<V2), are compared to find the greatest value, which is V2 in this case. Thus, the initial point A shown in FIG. 7 is shifted upward along the graph to a point B that corresponds to the greatest voltage V2. Thereafter, such a series of steps is repeated until the maximum power point MP1 is reached. The hill climbing technique is disclosed in JP-A-2001-325031, for example.
The conventional hill climbing method described above is found disadvantageous in the following points.
When all the solar cells SC are equally irradiated by the sun, a one-humped curve such as the one illustrated in solid in FIG. 7 will appear. However, the state of the solar irradiation onto the solar cells changes (a shadow is cast over a portion of the panels, for example), the voltage-power characteristic is varied, and a two-humped curve for example may appear, as shown in broken lines in FIG. 7. The illustrated two-humped curve has two local maximums: a lower maximum LP and a higher maximum MP2. In this case, if a point C is chosen for the starting point in performing the above-described hill climbing method, the real maximum point MP2 can be reached. However, if the point A is chosen for the starting point, the tracking will end when the local maximum point LP is reached, which is not the real maximum point (which is point MP2).
As readily understood, the situation will become worse when the voltage-power curve has more than two humps (that is, local maximums). In such a case, the conventional hill climbing method is powerless for finding the maximum power point, and hence the power generation system fails to operate in the maximum power mode.